Recent advances in the performance of such components are due in particular to the relatively easy fabrication of semiconductor multilayer structures in the standard MBE (molecular beam epitaxy) system, i.e. the GaAs/Ga(1-x)AlxAs system. By adjusting the growth parameters, the thickness of the quantum wells and the fraction x of aluminium in the barriers imposing the confinement potential, a narrow detection band (about 1 micron in width) may be chosen to be centred on a given wavelength.
This type of structure has the advantage of providing very good sensitivity because of the discretization of the energy levels within the conduction bands of the photoconductive materials used.
Thus, multiple-quantum-well detectors are recognized as providing a very good technical solution for fabricating matrices sensitive to infrared radiation within the 8-12 μm band.
In the context of inter-sub-band transitions, in order for this type of transition to be possible it is necessary for the electric field of the incident electromagnetic wave to have a component along the growth direction of the layers, said direction being perpendicular to the plane of the layers. The consequence of this physical effect is that a detector exhibits little absorption in the case of illumination at normal incidence.
It has already been proposed to use coupling means of the diffraction grating type (cf. Goossen and Lyon, Appl. Phys. Lett. 47, 1257-1259 (1985)) for generating said perpendicular component by creating diffracted radiation. Thus, a diffraction grating operating in reflection may be etched on each pixel (the detectors are back-lit) as described in the article “Grating-coupled quantum-well infrared detectors: Theory and performance”, J. Y. Anderson and L. Lundqvist, J. Appl. Phys. 71, 3600 (1992) and illustrated in FIG. 2, which demonstrates the use of arrays of studs for coupling the incident radiation whatever its polarization, through a view, performed by an electron microscope, of pixels about 20 microns in width, said pixels having, on their surface, diffraction gratings, the upper ring being merely an element for attachment to a matrix of read circuits on silicon.
The assembly formed by the matrix of pixels produced within the multiple-quantum-well structure and by the diffraction gratings is called hereafter the “focal plane”.
In general, a multiple-quantum-well structure makes it possible to produce layers, and therefore detectors, which are sensitive in very narrow wavelength ranges of the order of 10% (a wavelength variation Δλ/λ of the order of 10%).
However, this type of active structure does not allow the production of imaging devices capable of operating within broad wavelength spectra despite the very high sensitivity that they nevertheless can achieve.